1. Field of the Invention
This invention relates to automatic test equipment of the type used to test integrated circuits either singly or in combination with other circuits, and in particular, to a technique for deskewing signals supplied to a device being tested in such a system.
2. Description of the Prior Art
Numerous types of automated test equipment for the testing of individual integrated circuits or groups of integrated circuits are known. For example, the Fairchild Test Systems Division of Fairchild Camera & Instrument Corporation, assignee of this invention, manufactures systems known as Sentry.RTM. test systems. In such a system an individual integrated circuit is positioned in a suitable fixture to permit application of stimuli signals to various pins of the device, and reception of the resulting output signals from the device. By comparing the resulting output signals with those known to be produced by a satisfactory device, or expected by circuit analysis and calculations or other analytical techniques, the functionality and/or performance of the device being tested may be determined. Typically in such systems, a digital computer is used to control a timing module which supplies a variety of timing signals to a format control. The format control, in response to the timing module, generates signals of appropriate waveform and supplies them to a series of pin electronic circuits, each associated with a pin of the device under test. Signals from the device under test are returned to the pin electronic circuits and to a failure response unit for detecting the functionality and/or performance of the device being tested.
As increasing numbers of functions are placed on single integrated circuits or groups of integrated circuits, and as the performance of such circuits improves, the performance of the test system itself must be improved to enable it to detect variations in the performance of the integrated circuit being tested. One well-known problem in the manufacture and use of automatic test equipment is the timing skew of both the input signals supplied to the device under test, and the output signals received from the device under test. If the stimulation signals are not deskewed, the proper functioning and/or performance of the device being tested cannot be determined. In typical prior art systems, deskewing was accomplished using extensive manual adjustments of potentiometers associated with each pin of the automatic test equipment. In one prior art 120 pin system, each pin has 8 potentiometers associated with it for deskewing various signals supplied to, or received from, that pin. Thus, almost 1,000 potentiometers had to be manually adjusted in order to suitably align the system to perform tests. Because the settings of the particular potentiometers affected each other, it was often necessary to adjust each potentiometer more than once during alignment of the system. Obviously this was a lengthy, labor intensive, and expensive operation.
Another problem in prior art test systems is the generation and deskewing of inverted waveforms. In some modes of operation of automatic test equipment it is desirable to supply first one waveform, and then its complement, to the device under test. As explained above it is desirable that both such signals be deskewed.
Furthermore, with the operational speed of individual test systems approaching 20 megaHertz, skew tolerances of no more than +1 nanosecond maximum and +500 picoseconds typical are necessary. Using conventional prior art techniques, tolerances to within one nanosecond have been achieved, but only if tests are conducted immediately after alignment of the test system and only using small subsets of the system's timing and format capabilities. As the system is continuously used, the reliability of the alignment diminishes. Consequently, when a test engineer determines that yields of the devices being tested are varying, he does not know whether there is indeed a yield variation, or whether the test equipment has deviated from specifications.